Rapid light curves: A powerful tool to assess photosynthetic activity

Rapid light curves provide detailed information on the saturation characteristics of electron transport, as well as the overall photosynthetic performance of a plant. Rapid light curves were collected from samples of Zostera marina grown under low and high-light conditions (50 and 300 μmol photons m−2 s−1) and the distinctive patterns of RLC parameters are discussed, in terms of differential sink capacity and PSII reaction centre closure. Derived cardinal points of a rapid light curve (α, Ek and rETRmax) describe the photosynthetic capacity of a seagrass leaf, its light adaptation state and its capacity to tolerate short-term changes in light. The shapes of the corresponding F and F′m curves also provide information on the development of the trans-thylakoid proton gradient and thermal energy dissipation. Low-light leaves showed limited photosynthetic capacity and reduced activity of non-photochemical quenching pathways, whereas photosynthesis of high light leaves were not limited and showed an elevated level of non-photochemical quenching, possibly associated with xanthophyll cycle activity. Light-dark kinetics are also discussed in relation to relaxation of non-photochemical quenching and its various components. A curve fitting model is recommended based on the double exponential decay function. In this paper, we explain the fundamental aspects of a RLC, describe how it reflects the response to light exposure of a leaf, how to interpret these curves, and how to quantitatively describe and compare RLCs.